[qtwebkit] Disable persistent SG/GL when WebView is used.

It is not working and this prevents webkit from invalidating itself
which will result in blank views and potentially crashes.

The persistent SG / GL toggle whether the scene graph and
opengl context is released when the window is hidden. Starting
with the 5.2 update, all QML applications have these flags
set.
Merge pull request #19 from sletta/non-persistent

[qtwebkit] Disable persistent SG/GL when WebView is used.